Current semiconductor device technology requires fabrication of very small structures on the semiconductor substrate. In turn these structures are fabricated using photolithographic process. In this process light is projected though a mask to produce an aerial image which is then directed to a photoresist layer coated on semiconductor substrate thereby exposing the photoresist layer. After a developing process, an image is formed in the photoresist layer. This image is used as an etch mask to produce the structures on the semiconductor wafer.
Typically these structures are of the order of 0.18 to 0.25 micron in minimum dimension and smaller minimum dimensions can be expected as semiconductor technology advances. At these dimensions light interference phenomenon become important considerations and special masks are required to take this interference phenomenon into account to allow reproduction of the mask image in the developed photoresist layer accurately and without distortions. One such type of mask is called an attenuated phase-shifting mask (APSM). In such a mask an image is formed in a partially transmissive layer on a transmissive substrate. Light passing through the partially transmissive layer is 180.degree. out of phase from light passing through only the transmissive substrate. This results in destructive interference at the edges of the aerial image, giving a sharper edge and truer reproduction of the mask image. A drawback to this type of mask is an increase in the intensity of the secondary or side lobes of the aerial image, which if they become large enough can result in exposure of resist adjacent to certain image edges. Another consideration that needs to be taken into account is as image sizes decrease, the intensity of light needed to expose small images increases, but the intensity of light required to expose larger images does not increase as rapidly. This means larger images can be overexposed. While simple overexposure can often be accounted for by mask image size compensation, side lobe exposure can not. In attenuated phase-shift masks, when small images are exposed properly large images can often be seen to have additional adjacent images called side lobe images after development. This is illustrated in FIGS. 1 though 3.
FIG. 1 is a plan view of a portion of a conventional attenuating phase-shift mask. The mask is comprised of transmissive substrate 10 having a partially transmissive attenuating phase-shift region on transmissive substrate 10. Partially transmissive attenuating phase-shift region 12 contains mask images 14 and mask images 16 and 18. As depicted mask images 14, 16, and 18 are transmissive clear spaces although many of the concepts described here are applicable to opaque images also.
FIG. 2 is a plan view of a portion of a photoresist coated wafer exposed with the mask of FIG. 1. FIG. 3 is a partial cross-sectional view of the photoresist coated wafer of FIG. 2 through section AA of FIG. 2. Wafer photoresist layer 22 has been coated on top surface 21 of wafer 20. Images 24 have been formed in region 25 of wafer photoresist layer 22 and images 26 and 28 formed in region 29 of wafer photoresist layer 22. As can be seen side lobe images 26A have been formed along two sides of image 26 and side lobe images 28A and 28B have been formed along four sides of image 28 in photoresist layer 22. Side lobing can be a major concern when the larger images become approximately three times larger in at least one dimension then the smaller images. For example if images 24 are 0.4 by 0.4 micron in size and image 26 is 0.4 by 1.2 micron in size, and image 28 is 1.2 by 1.2 micron in size, the significant side lobing shown in FIGS. 2 and 3 will result. Considering that many kerf images for structures such as alignment marks and measurement marks (such as used for overlay) are very large compared to the smallest device images, side lobe images are produced that make edge detection of the instruments that use these marks very difficult and inaccurate. Side lobes also create problems on very large critical device structures.
Turning to the prior art, U.S. Pat. No. 5,589,303 to DeMarco et. al., teaches how an attenuating phase-shifting optical lithographic mask is fabricated, in a specific embodiment of the invention, by first depositing a uniformly thick molybdenum silicide layer on a top planar surface of quartz. The molybdenum silicide layer has a thickness sufficient for acting as an attenuating (partially transparent) layer in a phase-shifting mask. A uniformly thick chromium layer is deposited on the molybdenum silicide layer. The chromium layer has a thickness sufficient for acting as an opaque layer in the phase-shifting mask. Next, the chromium layer is patterned by dry or wet etching, while the chromium layer is selectively masked with a patterned resist layer. Then molybdenum silicide is patterned by dry or wet etching, using the patterned chromium layer as a protective layer, whereby a composite layer of molybdenum silicide and chromium is formed having mutually separated composite stripes. Any remaining resist is removed. Next the top and sidewall surfaces of some, but not others, of these mutually separated stripes are coated with a second patterned resist layer. Finally, the chromium layer, but not the molybdenum suicide layer, is removed from the others of the mutually separated composite stripes.
A drawback to the prior art methods is the difficulty of repairing the mask after fabrication as repair of attenuating phase-shifting layers tends to introduce other undesirable defects, and attenuated phase-shift masks are particularly difficult to repair after fabrication because of the need to keep the material in the region of the repair the same thickness as the surrounding material, and of the same composition in order to match phase-shifting capability and light transmission.
U.S. Pat. No. 5,506,0809 to Adair et. al., (hereby incorporated by reference) teaches a method of forming a substantially defect-free mask for optical and phase-shift lithography. The method involves depositing a transfer layer on a mask layer deposited on a transmissive substrate, forming in the transfer layer a mask image to be defined in the mask layer, inspecting the image formed in the transfer layer, repairing the image formed in the transfer layer, and transferring the corrected image from the transfer layer into the mask layer. The repair of the transfer layer is accomplished by removing unwanted portions of the transfer layer followed by filling any unwanted voids therein with selected material. Preferably, the fill material has the same desirable etching and/or optical characteristics as the surrounding transfer layer. However, any material that is substantially opaque to the radiation used to transfer the image from the transfer layer to the mask layer or resistant to the etch chemistries used can be successfully employed.
Accordingly, there exists a need for an improved method of fabricating an attenuating phase-shift mask that solves the side lobe imaging problem and ameliorates the need for post fabrication repair.